Today's communications systems utilize various types of cable and radio interfaces. The most reliable are glass optical fibres which also enable very high transmission rates. On the other hands, copper cables still form part of the telephone lines which are also used for transmission of data. Especially in the last decades, wireless communications has developed rapidly. All these data transport media have their own characteristics and are suitable for deployment in different scenarios and architectures.
Glass optical fibres (GOF) are used nowadays especially for communication requiring a very high bandwidth and very low attenuation. Since glass optical fibres have very small diameters and low numerical apertures (NA) its installation requires special and expensive connector tools and skilled installation workers.
Another possibility is the deployment of plastic optical fibres (POF), for instance, based on poly-methyl-methacrylate (PMMA) with a larger core diameter (about 1 mm) and a high numerical aperture (NA of approximately 0.3 to 0.5). The least expensive and most used plastic optical fibre is an SI-POF with a numerical aperture of 0.5. However, there is also an SI-POF with a low numerical aperture of 0.3 enabling higher data rates as well as PMMA GI-POF with a bandwidth length product near to 1 GHz×100 meter. PMMA has several attenuation windows that enable POF to be used with different visible light sources from blue to red Light Emitting Diodes (LED) or red Lasers Diodes (LD).
In comparison with GOF, plastic optical fibres have an advantage of a very easy installation. They can be deployed by professional or non-professional installation workers using basic tools such as scissors or cutters and inexpensive plastic connectors. It is resilient to misalignment and strong vibrations so it can be installed in industrial and automotive environments without loss of communication capacity. The POF connections have also much higher tolerance to residual dust on the terminal faces than GOF, due to the larger core diameter.
Since the transmission over POF is optic, plastic optical fibres are completely immune to electrical noise. Thus, the existing copper wiring will not interfere with data passing through plastic optical fibres so it can even be installed next to electrical cabling. Plastic optical fibre connectors and opto-electronics for POF are mainly low cost consumer parts which enable installation workers to save cable costs and installation, testing, and maintenance time. Plastic optical fibres have been widely employed, in particular, for infotainment networks in cars and can now be seen as a global standard for high-speed on-board car networks such as Media Oriented Systems Transport (MOST).
FIG. 1 illustrates an example of a system for transmission and reception of data over POF. The transmission over plastic optical fibres is based on a light intensity modulation with direct detection. The signal to be transmitted is generated from a digital circuit 110 for encoding and modulating the user bit stream information and passed to a transmitter (Tx) analogue front end (AFE) 120 for conversion of digital data into an electrical signal for controlling the light emitting element 130. After this conversion of the electric signal to an optical signal, the latter is then input to the optical fibre 150. Electrical optical converters used for plastic optical fibres are typically light-emitting diodes (LED) characterized by properties such as a peak wavelength, a wavelength width or launching modal distribution. The LED response in terms of electrical to optical conversion is non-linear. Therefore, the LED introduces harmonic distortion in the form of dynamic compression over the communication signal. Furthermore, the non-linear response has a high dependency with the temperature.
During the transmission of the signal via plastic optical fibres 150, the light is affected by severe attenuation as well as distortion mainly due to modal dispersion. The modal dispersion is caused by different modes of light propagating in the fibre on different paths and with different speeds and attenuations, resulting in different arrival times at the receiver. The optical signal is also affected by a so-called mode coupling where the energy of higher order modes is transferred to lower order modes and vice versa. As a consequence, an optical pulse is broadened which leads to lower the signal bandwidth.
At a receiver, the optical signal from the plastic optical fibre 150 is converted into electrical intensity by means of an opto-electric converter 170 such as a photodiode. Then, the electrical signal is processed by the analogue front end (AFE) 180. In particular, it is amplified, inter alia by a trans-impedance amplifier (TIA) and connected to a digital receiver 190. The TIA is typically the most important noise source which limits the final sensitivity of the communication system. Because POF presents a high attenuation factor with the length, the photodiode and TIA must be designed to be able to work with a very high range of optical power input, with limited voltage supply. This is allowed by implementing Automatic Gain Control (AGC) that controls the trans-impedance as a function of the photodiode average current. Several parameters, as harmonic distortion, bandwidth and delay group, as well as the input referred noise and flicker noise of the TIA depends on the variable trans-impedance, therefore the digital receiver must be able to track all these variable parameters in order to optimally decode the communication data.
Regarding the data transmission technology, GOF have been successfully using a non-return-to-zero (NRZ) modulation. In particular, current glass fibre communication systems mainly utilize NRZ 8b/10b or NRZI 4b/5b line coding which requires a baud rate of 1.25 GHz and 125 MHz for 1 Gbps and 100 Mbps solutions, respectively. Current plastic optical fibre solutions thus also adopted NRZ modulation for data communications. However, plastic optical fibres have a frequency and time response different from that of glass fibres and also have considerably higher attenuation. As a communication medium, plastic optical fibres show a very high modal dispersion due to its important differential mode delay and differential mode attenuation. The large area photodiodes required for coupling with a fibre typically have a limited bandwidth. In view of a plastic optical fibre frequency response, solutions supporting 100 or 150 Mbps are possible up to ca. 30 meters with enough link budget for installation; but 1 Gbps does not seem to be achievable without a more advanced technology.
FIG. 2A shows a variation of POF optical bandwidth (y axis, in MHz) as a function of the fibre length (x axis, in meters). FIG. 2B shows the variation of the bandwidth-length product (y axis, in MHz·100 m) as a function of the fibre length. Here, the fibre is an SI-POF with a numerical aperture NA of 0.5 (in particular, model Mitsubishi Eska-GH4001), and the light source is an RCLED with launching condition FWHN NA of 0.31, wavelength peak of 658 nanometers and an FWHN wavelength width of 21 nanometers. As can be seen from FIG. 1, a suitable flat response for a desired 1.25 GHz baud rate is only possible in the very first meters of the plastic optical fibre. For a laser light source, the optical bandwidth as a function of length is very similar. Therefore, the bandwidth bottleneck is produced by plastic optical fibres independently on how fast the light source is because the limiting factor is, in particular, the modal dispersion by mode coupling in the fibre.
As can be seen from the above described characteristics of the plastic optical fibre and the opto-electronics, its temperature and time-variant non-linear characteristics pose several challenges for optimization of data transmission over this medium. Techniques such as Tomlinson Harashima Precoding, adaptive equalization, adaptive coding and modulation help improving the transmission. However in order to efficiently employ them, additional information is to be transmitted with the data over the plastic optical fibre.
Standard IEEE 802.3u is known as fast Ethernet. Fast Ethernet may be transmitted according to 100BASE-FX over optical fibre, which may be a single-mode fibre (SMF) or a multi-mode fibre (MMF). Fast Ethernet provides transmission with rate of 100 Mbps at physical layer. It employs PCS and PMA (cf. IEEE 802.3 Clause 24, PMD: IEEE 802.3 Clause 26). 100BASE-FX does not provide a physical frame structure which would enable transmitting signals necessary for adaptive equalization, coding and modulation. The physical layer is based on the line block code 4b5b with NRZI (non return to zero inverted) modulation. The 4b5b code is a run-length limited code which maps groups of four bits onto groups of five bits. The 5-bit output words are predetermined in a dictionary and chosen to ensure presence of at least two transitions per block of 5 bits. The NRZI modulation codes binary 1 with a transition and binary 0 with no transition of a signal. The combination of NRZI and 4b5b provide a enough number of clock transitions per time, making easier the clock recovery. Free codes from the 4b5b coding are used for fail and collision signaling between the link partners. The bit runtime is further limited, so that the DC unbalancing is constrained. Moreover, the NRZI coding produces high frequency pre-emphasis, which aids to counteract the low pass response of the communication channel. The 4b5b line coding results in 25% extra required bandwidth.
Another standard is IEEE 802.3z (1000BASE-X), which provides 1 Gbps Ethernet over optical fibers (both SMF and MMF). Similarly as above, PCS and PMA are used (cf. IEEE 802.3 Clause 36, PMDs: Clause 38, for long (1000BASE-LX) and short waves (1000BASE-SX) lasers). It does not provide a frame structure for advanced modulation and equalization techniques. This standard employs 8b10b line coding with NRZ modulation. The 8b10b coding provides good DC balance and the limited run-time that makes easier the clock recovery in the receiver. Free codes from the 8b10b coding are used for signaling, carrier sensing, collision detection etc. However, a 25% extra bandwidth is required due to the line coding. Use of this standard for 1 Gbps over POF provides a very limited performance, being only possible in very short fiber (a few meters).
The standards used for fast transmission of signals over other media such as IEEE 802.3ab, 1000BASE-T (1 Gbps Ethernet over 4 twisted copper pairs Class D with nominal impedance 100 ohm IEC 11801:1995) are not suitable for plastic optical fibre since the plastic optical fibre has substantially different characteristics, although they may include a frame and differentiated symbols for training and normal data transmission. The plastic optical fibre is a medium on which optoelectronics typically presents even and odd orders harmonic distortions due to submicron technology limitations. In general, LED is a low-cost light source, with limited bandwidth and high non-linearity in the electrical current to optical power conversion. POF is linear for the typical injected power, which is limited due to eye safety constraints. Photodiode and Trans-Impedance Amplifier are highly dependent in bandwidth and noise on the gain. They must work in a very wide dynamic range (short and long fibers), so there are technological limits to provide a linear response. Typically there will be odd order harmonic distortion produced by these devices that require compensation. Additionally, the harmonic distortion in optoelectronics devices has a great dependence with temperature. This imposes the requirement of continuous tracking of the non-linear channel response.